The present invention relates to a scanning electronic microscope (SEM) for a step of controlling the yield of wafers.
An important step for controlling the yield of wafers has been to detect abnormal regions on the surface of wafers and to identify regions having process-related defects from information based on the observation of the features thereof and the analysis of elements.
Such detection of abnormal regions and observation of features have conventionally been carried out with optical wafer surface inspectors utilizing scattered laser beams, inspectors of foreign substances on wafer surface such as optical wafer surface inspectors which perform die-to-die comparison, review stations and the like. A review station is an optical observation apparatus of foreign substances which is linked with a defect and foreign substance detector when used.
However, as wiring patterns on wafers become finer, it has become impossible to observe closely features of abnormal regions to be observed with a laser system when they are in sizes of 0.3 xcexcm or less.
Conventional scanning electronic microscopes for observing the features on wafers have been scanning electronic microscopes having a stage for the observation of wafers added with automatic wafer loader, and abnormal regions have been observed with the following steps. (A3) among the following steps has been automatically carried out by a CPU. A1 through All below are steps for inspecting a single wafer as a unit.
(A1) A wafer is loaded into a scanning electronic microscope.
(A2) An inspector of foreign substances on wafer surface acquires information on features on the wafer and information on abnormal regions.
(A3) A coordinate transformation coefficient between the coordinate systems of a wafer surface inspector based on the recognition of wafer outline and the scanning electronic microscope is automatically obtained according to Japanese Patent Publication Tokkaihei-06-174644 xe2x80x9cMethod for Automatically Setting Coordinate Transformation Coefficientxe2x80x9d.
(A4) The coordinate positions of alignment marks on the wafer are observed to obtain a coordinate transformation coefficient between the coordinate systems of the wafer surface inspector and the scanning electronic microscope according to Japanese Patent Publication Tokkaihei-06-258240 xe2x80x9cMethod for Coordinate Transformationxe2x80x9d.
(A5) The stage is moved to the positions of abnormal regions identified by the inspector of foreign substances on wafer surface.
(A6) Adjustment of the lens system including focusing is carried out to acquire electron beam images or optical images.
The present invention relates to a scanning electronic microscope (SEM) and to techniques for automatically observing features on a semiconductor wafer to control the yield of the semiconductor wafers.
(A7) The abnormal regions are recognized.
(A8) The stage is moved to the positions thereof.
(A9) Magnifications in accordance with the size of the abnormal regions are set to acquire electron beam images or optical images.
(A10)) The process proceeds to the next step if the acquisition of images of all abnormal regions to be observed is completed and, if not, the process continues from step (A5) on the next abnormal region.
(A11) The wafer is unloaded from the scanning electronic microscope to terminate the process.
A problem arises here in that a scanning electronic microscope for observing features on a wafer has throughput which is slower than a laser system by a factor of several dozens and is backward in automation.
As a solution to this, there has been a need for full automation of conventional scanning electronic microscopes for observing features on wafers and improving operator throughout.
In order to solve the above-described problems, information on wafers to be observed is acquired from an operator, and steps for observing features on the wafers are sequentially and automatically carried out by a CPU.
The observation of repetitive patterns such as those of a memory cell region of a DRAM (hereinafter referred to as xe2x80x9crepetitive pattern observation methodxe2x80x9d) is carried out with the following steps. The steps other than (B1) are all automatically performed by a CPU.
(B1) Information on the type of wafers to be observed and information on queries from an inspector of foreign substances on wafer surface regarding the wafers is acquired from an operator. The information on the type of wafers to be observed is the size of the wafers (e.g., 6 inches and 8 inches), origin marks on the wafers (e.g., an orientation flat and a notch) and the like. The information on queries from the inspector of foreign substances on wafer surface is device IDs, process IDs, lot IDs, slot IDs and the like.
C1 through C8 described below are steps preceding alignment which are a pre-process.
(C1) Wafers are loaded into a scanning electronic microscope in accordance with the information on the type of wafers to be observed.
(C2) Information on the positions and sizes of abnormal regions is acquired from the inspector of foreign substances on wafer surface in accordance with the information on queries from the inspector of foreign substances on wafer surface.
(C3) A coordinate transformation between the coordinate systems of a wafer surface inspector based on the recognition of wafer outline and the scanning electronic microscope is automatically obtained according to Japanese Patent Publication Tokkaihei-06-174644 xe2x80x9cMethod for Automatically Setting Coordinate Transformation Coefficientxe2x80x9d.
(C4) Movement is made to the positions of alignment marks or defects at a predetermined distance from the alignment marks. Specifically, the stage is moved such that the positions of the alignment marks or the features at a predetermined distance from the alignment marks are centered in the field of view. Corners of the die or cross marks are frequently used as the alignment marks.
(C5) Automatic adjustment of the lens system including automatic focusing is carried out to acquire electron beam images or optical images.
(C6) The stage positions of the features are recognized from the images.
(C7) The process proceeds to the next step if the observation of all of the alignment marks or the features at a predetermined distance from the alignment marks is completed and, if not, continues from (C4) for the next alignment mark. The alignment marks or the features a predetermined distance from the alignment marks are normally measured at three points or more.
(C8) A coordinate transformation coefficient between the coordinate systems of the wafer surface inspector and the scanning electronic microscope is obtained by observing the coordinate positions of the alignment marks on the wafer using the above-described stage position according to the Japanese Patent Publication Tokkaihei-06-258240 xe2x80x9ccoordinate transformation methodxe2x80x9d. D1 through D7 below are steps for acquiring images of abnormal regions using the repetitive pattern observing method.
(D1) The stage is moved such that the abnormal regions whose positions and sizes have been acquired from the inspector of foreign substances on wafer surface are centered in the field of view.
(D2) Automatic adjustment of the lens system including automatic focusing is carried out to acquire electron beam images or optical images as images of the abnormal regions (image acquisition at a low magnification).
(D3) Partial images of the abnormal regions are recognized from the images of the abnormal regions.
(D4) Visual inspection is carried out to classify the abnormal regions. That is, shorts (shortcircuits), breaks (breakage), scratches and the like are identified.
(D5) The stage is moved to positions where they are detected.
(D6) Magnifications in accordance with the sizes of the detected abnormal regions are set to acquire electron beam images or optical images (image acquisition at high magnifications).
(D7) The process proceeds to the next step if the acquisition of all of the images of the abnormal regions to be observed is completed and, if not, continues from (D1) for the next abnormal region.
(E1) The wafers are unloaded from the scanning electronic microscope.
(E2) The images acquired on the abnormal regions, information on the classification and the like are provided to the operator.
The observation of features other than repetitive patterns (referred to as die-to-die observation method) is carried out with the following steps.
The above-described (B1)) through (C3) are carried out.
F1 through F9 below represent steps for acquiring images of abnormal regions by the die-to-die observation method.
(F1) The stage is moved to the positions on an adjacent die (a unit of chips on a wafer) which correspond to the position of the abnormal regions acquired by the inspector of foreign substances on wafer surface.
(F2) Automatic adjustment of the lens system including automatic focusing is carried out to acquire electron beam images or optical images as reference images. The dies on a wafer are all in the same configuration. A movement is made to regions of a die other than the dies having abnormal regions at the same distance from the origin of the die, and images acquired there are images having no abnormal region, i.e., reference images.
(F3) The stage is moved to the positions of the abnormal regions acquired by the inspector of foreign substances on wafer surface.
(F4) Automatic adjustment of the lens system including automatic focusing is carried out to acquire electron beam images or optical beams as images of abnormal regions.
(F5) Partial images of the abnormal regions are recognized from the reference images and images of the abnormal regions.
(F6) The abnormal regions are classified.
(F7) The stage is moved to the positions of detection.
(F8) Magnification is set in accordance with the sizes of the detected abnormal regions to acquire electron beam images or optical images.
(F9) The process proceeds to the next step if the acquisition of images of all the abnormal regions to be observed is completed and, if not, continues from (F1) for the next abnormal region.
The above-described (E1) and (E2) are carried out.
The recognition of the stage position at the above-described (C6) is carried out according to the following two methods.
The first method is a method wherein the stage positions of alignment marks on a wafer or features at a predetermined distance from the alignment marks are automatically recognized using a normalized correlation coefficient from electron beam images or optical images obtained therefrom or images differentiated therefrom, the method being specifically carried out with the following steps.
(G1)) Alignment marks or features at a predetermined distance from the alignment marks are selected as features that can be easily recognized.
(G2) The positions of the above-described features on the inspector of foreign substances on wafer surface and electron beam images and optical images acquired from the features are registered in advance.
(G3) Those images and images acquired at (C5) or differential images thereof (images which are bright only along the contour thereof obtained with a Sobel, Laplacian or other type filter that utilizes differences between adjacent pixels) are matched according to the normalized correlation coefficient method.
The matching mentioned here is a method for finding a normalized correlation coefficient for overlaps between images obtained by shifting the images acquired at (C5) and the images acquired at (G2) and for finding an amount of shift which maximizes that value.
A normalized correlation coefficient can be expressed by the following equation.
For example, a normalized correlation coefficient for a two-dimensional array Xij and a two-dimensional array Yij is:
xcexa3(Xijxe2x88x92Xmean)(Yijxe2x88x92Ymean)/{xcexa3(Xijxe2x88x92Xmean)(Xijxe2x88x92Xmean)xcexa3(Yijxe2x88x92Ymean) (Yijxe2x88x92Ymean)}
where, xcexa3 represents the sum of all i and j, and xe2x80x9cmeanxe2x80x9d represents the average of the same.
The numerator xcexa3(Xijxe2x88x92Xmean) (Yijxe2x88x92Ymean) is a correlation coefficient, and the denominator {xcexa3(Xijxe2x88x92xmean)(Xijxe2x88x92xmean) xcexa3(Yijxe2x88x92Ymean) (Yijxe2x88x92Ymean) } is a coefficient for normalization.
(G4) The amount of shift found as a result of the matching is converted into a distance in the coordinate system for the stage position.
(G5) The sum of this distance and the stage position as a result of the movement at (C4) is recognized as the stage position for the feature.
The second method is a method in which linear features or circular features are detected from electron beam images or optical images acquired from alignment marks on a wafer or features at a predetermined distance from the alignment marks and in which the stage positions of the features are automatically recognized. It is specifically carried out with the following steps.
(H1) Alignment marks or features at a predetermined distance from the alignment marks are selected as features that can be easily recognized.
(H2) The positions of the above-described features on the inspector of foreign substances on wafer surface and electron beam images and optical images acquired from the features are registered in advance.
(H3) Linear images, circular images and the like are detected from those images and images acquired at (C5) using a method such as Hough conversion. Hough conversion is a technique to convert a dot sequence into a line segment. A line segment can be detected even from a dot sequence having discontinuation.
(H4) An amount of shift is found based on the correspondence of the linear features and circular features.
(H5) It is converted into a distance in the coordinate system of the stage position.
(H6) The sum of this distance and the stage positions as a result of the movement at (C4) is recognized as the stage positions of the features.
At the above-described (D3), a partial image of an abnormal region is recognized from an image of the abnormal region using a method xe2x80x9cin which a electron beam image or optical image which is a repetitive feature including an abnormal region is an image of the abnormal region; an image obtained by shifting the same images at a repetitive interval is a reference image; and the two images are compared to automatically recognize the partial image of the abnormal regionxe2x80x9d. Specifically, the following steps (J1) through (J6) are carried out.
(J1) As a pre-process, noises are eliminated with a smoothing filter or the like. Here, xe2x80x9cnoisesxe2x80x9d are pulse noises and the like (The S/N ratio of SEM is very poor).
(J2) The image of the abnormal region and the reference image are matched. Specifically, the reference image is shifted such that the value of the normalized correlation coefficient is maximized.
Here, the reference image is obtained by shifting the image of the abnormal region at a repetitive interval.
Here, the matching is to find a normalized correlation coefficient for an overlap between an image obtained by shifting the image of the abnormal region by the amount described below and the image of the abnormal region and to find an amount of shift that maximizes this coefficient.
The range of the amount of shift:
repetitive interval X (1xe2x88x92xcex1) to repetitive interval X (1+xcex1)
where xcex1 is a number greater than 0 and less than 1 which depends on an error of the repetitive interval.
The repetitive interval is obtained using an automatic method utilizing autocorrelation or a method utilizing a value input by an operator.
Autocorrelation is often used to check the periodicity of a pattern.
For example, autocorrelation r(a, b) for a two-dimensional array Xij can be expressed as follows.
xcexa3(Xijxe2x88x92Xmean)(Xi+a j+bxe2x88x92Xmean)/{xcexa3(Xijxe2x88x92Xmean) (Xijxe2x88x92Xmean)xcexa3(Xi+a j+bxe2x88x92Xmean) (Xi+a j+bxe2x88x92Xmean)}
where xcexa3 represents the sum of all i and j; xe2x80x9cmeanxe2x80x9d represents the average of the same; and r(0, 0) is 1.
For example, when xij is a pattern having periods, r(a period in the X-direction, a period in the Y-direction) is also 1.
Further, autocorrelation is related to the autoregressive method and power spectrum method.
(J3) Noises are eliminated by using each pixel in an image of an abnormal region as a reference pixel, finding the pixel having a value closest to the reference pixel in the neighborhood of a pixel in a reference image corresponding to that pixel and substituting the value for the pixel value in the reference image.
The noise elimination is carried out for each pixel of the image of an abnormal region with the following steps.
FIG. 4 is an illustration for explaining the method of eliminating noises in the reference image using each pixel in the image of an abnormal region.
A pixel XR of the reference image corresponding to a pixel XD of the image of an abnormal region is found. In FIG. 4, the density of the pixel XD is 59, and the density of the pixel XR is 252. Here, xe2x80x9cdensityxe2x80x9d is the value of a pixel. Here, it means an amount of secondary electrons which has been subjected to A-D conversion using data 0 through 255.
The density of the pixel XR or the pixel above, under, to the left or to the right of the same whichever closest to the density of the pixel XD is substituted for the density of the pixel XR. Referring to FIG. 4, 30 is chosen from among 25, 30, 252, 135 and 204.
It is apparent that this operation has eliminated 252 which is a peak noise.
(J4) A pixel of the abnormal region is detected using the sum of square of the difference between a pixel value of the image of the abnormal region and a pixel value of the corresponding reference image.
This process is carried out as follows for each pixel of the image of the abnormal region.
A rectangle (referred to as xe2x80x9cdetection rectanglexe2x80x9d) is formed which is centered at a pixel of the image of the abnormal region and a pixel of the reference image corresponding to the same. The lengths of the sides are empirically determined as parameters.
Pixels in the rectangle are paired to calculate an evaluated value P of the abnormal region as follows.
If P is smaller than a preset value, the pixel is a pixel of an abnormal region.
IR(X,y): a pixel value of the reference image
ID(X,Y): a pixel value of the image of the abnormal region (x and y are coordinates after matching)
N: the number of pixels in the detection rectangle
NT: the number of pixels in a rectangle where the image of the abnormal region and the reference image overlap
xcexa3: The sum in the detection rectangle
xcexa3T: The sum in the rectangle where the image of the abnormal region and the reference image overlap
mean: The average in the rectangle where the abnormal region and the reference image overlap
P=[xcexa3I{IR(X,Y)xe2x88x92IRmean}2N/NT]/[xcexa3{ID(X,Y)xe2x88x92IR(X,Y)}2]
(J5) Pixels of a ghost abnormal region are eliminated.
Since the reference image and the image of the abnormal region are identical, the pixel of the abnormal region is detected in two locations as shown in FIG. 5. One of them is referred to as xe2x80x9cghost abnormal regionxe2x80x9d. In other words, it is a normal pixel in an image of a foreign substance corresponding to a defective foreign substance in a reference image.
A description is made here on a method in which the pixel in the circle on the right hand side is a pixel in a ghost abnormal region. As processing steps for eliminating the pixel of the ghost abnormal region, the following process is carried out for each pixel of the image of the abnormal region.
An evaluated value PA of the abnormal region for a pixel XA of the image of the abnormal region and an evaluated value PB for a pixel XB apart therefrom by the amount of the shift of the image are found, and the value PA or PB whichever is greater is substituted for PA.
(J6) A partial image of the abnormal region is collectively formed by pixels of the abnormal region obtained at (J5).
In the above-described (F5), an electron beam image or optical image including an abnormal region is used as an image of the abnormal region; an image on a different die corresponding to this image is used as a reference image; and those two images are compared to recognize a partial image of the abnormal region automatically.
Specifically, the following steps (K1) through (K6) are carried out.
(K1) As a pre-process, noises are eliminated with a smoothing filter or the like.
(K2) The image of the abnormal region and the reference image are matched.
Here, the image of the abnormal region and the reference image are different images.
Here, the matching is to find a normalized correlation coefficient for an overlap between an image obtained by shifting the reference image and the image of the abnormal region and to find an amount of shift which maximizes this coefficient. While the amount of shift can be so large as to eliminate the overlap, a smaller amount of shift is desirable. Therefore, the absolute value of the amount of shift multiplied by a negative constant is added to the normalized correlation coefficient as a penalty.
(K3) The pixel values of the image of the abnormal region or reference image are rewritten such that the image of the abnormal region and the reference image have the same histogram to simulate the image.
(K4) Noises are eliminated by using each pixel in an image of an abnormal region as a reference pixel, finding the pixel having a value closest to the reference pixel in the neighborhood of a pixel in a reference image corresponding to that pixel and substituting the value for the pixel value in the reference image.
(K5) Pixel of the abnormal region are detected using the sums of square of the difference between pixel values of the image of the abnormal region and pixel values of the corresponding reference image.
A partial image of the abnormal region is collectively formed by pixels obtained at (K6) and (K5).
At the above-described (D4) and (F6), the type of an abnormal region is automatically classified based on quantities that characterize a partial image of the abnormal region recognized at (D3) and (F5) and quantities that characterize an image of the abnormal region and a reference image corresponding to the partial image. It is specifically carried out using the following method.
The quantities that characterize a partial image of an abnormal region include:
the area of the abnormal region converted from the number of pixels therein;
the ratio between the area of a rectangle approximated therefrom and the above-described area (The rectangle is approximated as follows. The directions of coordinate axes and values distributed along them are identified as in two dimensional analysis of major components, and a rectangle that covers about three times of the distributed values in the negative and positive directions is the rectangle to be obtained.);
the ratio between the long sides and short sides of the above-described rectangle;
the length of the boundary; and
the number of regions:
The quantities that characterize an image of the abnormal region and a reference image corresponding to the partial image include:
the ratio between the average luminance of the image of the abnormal region and the average luminance of the reference image;
the ratio between the luminance distribution of the image of the abnormal region and the luminance distribution of the reference image;
the correlation ratio between the image of the abnormal region and the reference image; and
the ratio between the sum of square of the gradient of the image of the abnormal region (the pixel values of a differentiated image thereof) and the sum of square of the gradient of the reference image.
A neural network or a membership function for which those characteristic quantities serve as variables is created to identify the type of the abnormal region automatically from the output value thereof.
The neural network or membership function is appropriately adjusted by an operator.
When accuracy of the stage is required, a method is used in which the stage position is moved to alignment marks or features at a predetermined distance from the alignment marks in the vicinity of observing positions when observing foreign substances or defects (referred to as xe2x80x9cabnormal regionsxe2x80x9d) as the stage is moved at (D1), (F1) and (F3), and the above-described features are recognized in the observing positions to correct any shift of the stage position automatically.